Ophiorrhiza is a notably species-rich, taxonomically complicated genus found in wet tropical forests of SE Asia (Darwin, Lyonia 1(2): 47-102. 1976); it has been little studied and is particularly poorly known in SE Asia (I. Schanzer, pers. comm.). Ophiorrhiza was studied for China by H. S. Lo (Bull. Bot. Res., Harbin 10(2): 1-82. 1990), who variously described 44 of the 68 Ophiorrhiza species recognized by Lo in FRPS (71(1): 110-174. 1999). The genus was studied in India by Deb and Mondal (Bull. Bot. Surv. India 39(1-4): 1-148. 1997), who recognized 47 species there. Darwin (loc. cit.) reported that the plants are notably variable in many vegetative features, many of which were shown by him not to be informative to separate species. H. S. Lo (loc. cit. 1999: 111) reported that the calyx and corolla lobes are occasionally 6, but this has not been noted by other authors nor seen on specimens studied; this may be an occasional variation found in one or two flowers on unusual plants, as in many Rubiaceae species. H. S. Lo also described the placentas as ascending from the septum base; however, Darwin (loc. cit.: 56) reported that in the Pacific Ophiorrhiza species the placenta is inserted in the middle of the septum in the flower and then often becomes displaced to near the base of the septum in fruit, whereas Puff et al. (Rubiaceae of Thailand, 190. 2005) gave the insertion of the placenta as being in the lower half of the septum. Puff et al. reported that the fruit function as splash cups for seed dispersal and observed that, regardless of the orientation of the flowers, the fruit become erect with enlarged and strengthened pedicels. Tan and Rao (Biotropica 13: 232-233. 1981) reported vivipary in a species of Ophiorrhiza growing in Singapore, with the seeds germinating within the capsules, pushing their cotyledons out through the suture where the valves open normally, and extending roots through the capsule locules and tissues; a similar condition may be found in Chinese Ophiorrhiza.

Darwin (loc. cit.: 47-102) noted that the presence of distyly in Ophiorrhiza has been controversial because the arrangement of the stigmas and anthers in the first dimorphic species found here differs from that of classic distyly, with strictly reciprocal sizes and positions. However, since then the recognition of distyly in Rubiaceae has expanded to include species that are at least a bit dimorphic and have intra-morph incompatibility, and Ophiorrhiza clearly belongs in this group and has been considered distylous by subsequent authors (Deb & Mondal, loc. cit.; Kudoh et al., J. Trop. Ecol. 17: 719-728. 2001; Schanzer, Thai Forest Bull. 33: 140-166. 2004). Also, some species with markedly dimorphic distylous flowers have subsequently been discovered (e.g., O. aureolina and O. rufopunctata). Deb and Mondal (loc. cit.: 15, f. 7) illustrated some of the variation in stamen and stigma position and internal corolla pubescence in distylous species of this genus. Homostyly has also been confirmed in the genus (Nakamura et al., J. Jap. Bot. 81: 113-120. 2006; J. Plant Res. 120: 501-509. 2007) and some species also appear to be autogamous (Nakamura et al., loc. cit. 2006). Schanzer (loc. cit.) noted that some species appear to vary in floral biology across their range, to include both homostylous and distylous populations; this situation has been found elsewhere in some distylous Rubiaceae, which have variation in expression of distyly (e.g., Faivre & McDade, Amer. J. Bot. 88: 841-853. 2001), though in those cases, the flowers resemble one of the distylous forms while Schanzer described a distinct floral form in the monomorphic plants. Schanzer also noted that some of these floral forms may be aberrant and cleistogamous rather than distylous. Nakamura et al. (loc. cit. 2007) studied two supposedly conspecific varieties of O. japonica in Japan, one homostylous and the other distylous, and concluded that the self-compatible homostylous plants differed in ploidy level, comprised a distinct lineage according to cpDNA sequences, and are better considered a separate species. They also noted that floral biology is not correlated with ploidy in general in Ophiorrhiza. Observations and documentation of the floral biology of Chinese Ophiorrhiza species are so far limited. Kudoh et al. (loc. cit.) presented a detailed analysis of the floral forms and possible genetic controls of these in O. napoensis in Guangxi, China. Also notable in the floral morphology of this genus is the apparent wide variation in corolla pubescence within a species, sometimes with the long-styled and short-styled flowers reportedly different (e.g., O. oppositiflora, floral forms similar but corolla pubescence variable, Deb & Mondal, loc. cit.: 88, f. 39; O. austroyunnanensis, pubescence apparently correlated with floral form, H. S. Lo, loc. cit. 1990: 31, f. 8).

Deb and Mondal (loc. cit.: 1) noted that the genus name alludes to the presumed healing properties of the root of these plants for snakebite and that Ophiorrhiza mungos and O. japonica are used for such in the Indian subcontinent. They also noted that species of this genus are used as medicine (for snakebites, stomach ulcers, skin eruptions, rheumatism, heart diseases), dye (red, for wool and hair), and food (the fruit), and list several references detailing their ethnobotany and medical chemistry.

The monotypic genus Hayataella, endemic to Taiwan, was recognized by several authors (particularly Taiwanese authors) as distinct from Ophiorrhiza, though it was synonymized by H. S. Lo (Bull. Bot. Res., Harbin 18: 276-277. 1998). Its morphology and molecular systematics were studied by Nakamura et al. (J. Plant Res. 119: 657-661. 2006), who concluded based on molecular data that the species belongs to a relatively derived clade of Ophiorrhiza and formally transferred the species, eliminating another of Taiwan’s few endemic genera.

H. S. Lo (loc. cit. 1990: 1-82) presented the definitive work on this genus in China. Most recently, Ophiorrhiza has been studied in China by Duan and Lin (Acta Phytotax. Sin. 45: 870-879. 2007), who synonymized several of Lo’s species. Their species circumscription is relatively broad compared to that of Lo, and a few of the species they synonymized are provisionally recognized here pending further study and a broader, consistent review of Ophiorrhiza in China.

Overall, the treatment of Ophiorrhiza here is primarily an organization of the published information, rather than a revisionary work. A few other species are keyed here even though their descriptions are incomplete; their placement is based in part on the key of H. S. Lo in FRPS (loc. cit. 1999: 112-117). In the FRPS treatment, H. S. Lo described in some detail the arrangement and degree of surface development and visibility of the tertiary venation on the abaxial leaf surface of many Ophiorrhiza species; however, this is incompletely described for the Chinese species, is variable within species, and was not used by Lo to distinguish species nor by other authors and, therefore, is not detailed here. Details of the anthers and stigmas are also incompletely described for Chinese Ophiorrhiza and mostly not used to separate species; the details available are summarized in the comments following the species description.

Among the names published in Ophiorrhiza by H. S. Lo (loc. cit. 1990), eight lacked an acceptable indication of type and were therefore not validly published under Art. 37 of the Vienna Code. In one case (O. chingii), two gatherings were cited but neither was indicated as the type, and in the other seven cases only one gathering was cited, which under Art. 37.3 is acceptable as indication of the type, but under Art. 37.6, on or after 1 January 1990, indication of the type must also include the word "typus" or "holotypus" or an equivalent in a modern language, and Lo did not include such words. All but one of the eight names were validated by S. Y. Jin and Y. L. Chen (Cat. Type Spec. Herb. China (Suppl.), 189-191. 1999) in each case by reference to Lo’s Latin description and by indication of a single gathering as the type (as "T."). The one remaining name, O. longicornis, is validated here.

About 200-300 species: tropical and subtropical Asia, Australia, New Guinea, Pacific islands; 70 species (49 endemic, one of unconfirmed occurrence) in China.